Process for the production of molybdenum dioxide



' ide, M003,

' 6 Claims. (or. 23-21 dUgine, Paris, France, a corporation of ABSTRACT OF THE DISCLOSURE A process for producing molybdenum dioxide by reacting, in stoichiometric amounts, temperature range between 600 C. and 700 C. and evolving therefrom free S0,, to form a reaction product, and then desulfurizing the reaction product in an atmos'-.

phere containing less than 10%. S at a temperature be tween 400 C. and 600 C. to obtain molybdenum dioxide, which is then permitted to cool to at least 250 C in neutral or reducing atmosphere.

This invention relates to prepartion of molybdenum dioxide for industrial uses'such as production of molybdenum steels and alloys. I l I Present industrial practices for addition of molybdenum to a bath of liquid steel include placing a quantity of a molybdenum alloy, generally ferromolybdenum, which has the. amount of molybdenum required within a solid charge of a furnace before the charge is melted. Another practice comprises adding the ferromolybdenum to the molten bath. Such practices have disadvantages due to the high melting point and great density of ferromolybdenum, which is slow to dissolve in the steel bath because the temperature of the bath is generally lower than the melting point of the ferromolybdenum. Accordingly, the ferromolybdenum remains on the bottom of the furnace and, after tapping out thesteel the furnace operator finds pieces of the ferromolybdenum on the bottom of the furnace. Consequently, the steel tapped out is deficient in molybdenum and residual molybdenum left in the furnace is liable to be incorporated into the next heat of steel in which it is unnecessary, or even unwanted, if the next heat requires a low molybdenum content. Thus, consecutive heats of steel may not havethe specified amount of molybdenum.

To overcome the foregoing problems, a differentpractice applies molybdic oxide, M00 which may or may not be accompanied by a reducing agent, onto a steel bath. The reducing action of the iron, and contingently that of the reducing agent, frees the molybdenum which becomes incorporated into the has disadvantages which arise fromthe molybdenum oxhaving a considerable vapor pressure at the temperatures of application. During its loading onto the surface of the liquid steel bath, a substantial proportion is lost by sublimation or by entrainment in the pulverulent condition. Inlight of the cost of molybdenum, a sizable loss is encountered. Moreover, the amount lost'is variable and the steel or alloy may lack the specified amount of molybdenum.

Use of molybdenum dioxide, M00 instead of M00 in this last method of operation ofiers considerable advantages. In the first place, M00 requires less reducing agent since it contains no more than 2 atoms of oxygen instead of 3 per atom of molybdenum.

In the second place, it is much less volatile than M00 as shown in the following table, wherein percentages repdElectro-Metallurgie et des Acieries molten bath. However, this method nited States Patent 0 MoO and M08, in a 3,33 ,100 Patented Aug. 15, 1967 iceresent the molybdenum losses from an identical weight of a solid product heated for 5 minutes within a given current of a gas. To extend'testing beyond this period serves Loss of M0 in percent of the Mo Present Inltlnlly (In the presence of argon or nitrogen) Temperature in C. I Moo. Moo.

This table thus demonstrates the dioxide in respect of loss by volatilization.

These advantages must be considered in the preparation of'the alloys having a high content of molybdenum,"in particular the ferromolybdenum alloys. Such preparation is performed at high temperature by the reduction of molybdic oxide, M00 by anactive reducing agent such as aluminum or silicon. Smoke containing a substantial amount of molybdic oxide which is sublimated is released in abundance during this reaction and the amount of molybdenum in the smoke represents approximately 4% of the weight .of molybdenum used in the preparation. The recovery of'these dusts requires installation of costly filters which can handle greatvolumes of gas to be purified within a short time. Such losses of Mo are very greatly reduced by replacing molybdic oxide with the dioxide, MOO 1 I Thus, it is not surprising that the use of M00 has been recommended for production of molybdenum steel and alloys. However, the different known processes for the production of M00; by the reduction of M00 with a gaseousreducing agent such as hydrogen, ammonia or illuminating gas, are too expensive. Also, direct reduction of M00, by a solid reducing agent such as C or MoS- results in too impure a product which contains residual C or S. In particular, the known processes which react M00 and M08 do not include direct preparation of a product having a low sulphur content. I I

:I have found .that at a temperature exceeding 400 C., themolybdenum-dioxide fixes a quantity of sulphur which increases with the partial pressure of the sulphur dioxide of the atmosphere in which it is table shows the sulphur contents of a technical product initially containing S==0.040% which was kept at 650 C. for one-half hour under atmospheric pressure in different atmospheres.

This fixing of sulphur by molybdenum dioxide does not depend only upon the partial pressure of sulphur dioxide, but also'upon the length of time of subjection of the dioxide to presence of certain impurities.

My invention relates to a process which includes a reaction of M05 and M00, and renders it possible-to produce a molybdenum dioxide, M00 which has a very low 8 content and a low M00 content. Specifically, the process comprises intimately mixing finely divided products containing M00 and M08, in stoichiometrical amounts which allow for contingently presentimpurities liable to react with these two compounds. The resulting mixture is very gradually heated within an enclosure wherein the advantage ofliered by the generated. The following the particular atmosphere and upon the' 3 atmosphere is conditioned. The reaction between M and M; takes place between 600 C. and 700 C., and preferably between 650 C. and 700 C., under a pressure close to and slightly above the atmospheric pressure during which reaction S0 evolves and forms the ing M05 However, my invention is also applicable toconstituents which, in addition, contain another oxidizing agent or another reducing agent, and are sources of M00 or M08 The relative quantities of the initial constituents introduced into the reaction enclosures will then be determined, while taking into account the nature and proportion of the oxidizing and/or reducing elements which accompany theMoO and M08 For example, conventional concentrates of molybdenite are not wholly free of the oils used to obtain them by flotation, and this fact should be considered that these oils exert a reducing action upon M00 in the process. Analogously, if the mixture introduced into the reaction enclosure is the product from a molybdenite roasting kiln, the sulphur it contains must be measured to determine the proportions of the products which are to be mixed in order to practice my process. Moreover, the operation of a roasting kiln of this kind may be regulated so that the product obtained may be used directly in my process.

The physical condition and chemical composition of the products introduced into the kiln or furnace are adapted to the quality of the molybdenum dioxide to be manufactured. In respect to granulometry in-particular, it is advisable to use molybdic oxide M00 whose grains are smaller than 2 mm. The flotation concentrates are appropriate for the M08 however, it is desirable that this M05 have a high degree of fineness. Additionally, special attention is devoted to stirring of the M00 and MOS; to obtain an intimate mixture.

The reaction between the M00 and the M08 is performed in a hermetic rotary furnace which isequipped with devices for conditioning the atmosphere therein. The reactive mixture is heated progressively and maintained at a temperature exceeding 650 C. for a certain time, such as several hours. Temperatures which exceed 700 C. are avoided so as not to efi'ectvfritting of the M00 and M08 The time of this heating or first stage depends upon the quality of the product one wishes to obtain, and upon certain characteristics of the reactive mixture, for example, grain sizes, mode of operation, the agitation of the mixture, or the thickness of a solid layer in the furnace. 1

During this first stage, the atmosphere of the furnace is under pressure in slight excess of that on the outside of the furnace (for instances 5 to 10 mm. of water) and is progressively enriched with S0; until the proportion of S0 exceeds 80%. Naturally, escape of. the gases to the outside occurs.

The yield from my process varies according to the operating conditions and may reach 95%.

Generally, this first stage is terminated when the reaction products contain less than 0.5% S and less than M003. However, my invention includes commencing the desulphurization, or second stage, earlier and also covers extending the first stage until release of S0 becomes insignificant, i.e. until the pressure inside the furnace stops being superior to the atmospheric pressure, thereby avoiding air entering the furnace.

The residual sulphur content results predominantly from the action of an atmosphere rich in S0 upon the M00 of the reaction product; half the residual sulphur I the M00 product while a very low percentage of S is results from this action when the S0 content in the atmosphere of the furnace ranges about Thus, it is advisable to desulphurize the reaction product, and to achieve same, the product is maintained at a moderately high temperature, 400 C. to 600C, within an enclosure under a pressure close to atmospheric pressure, in which partial S0 pressure is as low as possible and generally lower than 10%. In this enclosure, the

scavenging operation with an inert gas such as nitrogen or argon is one way of effecting the desulphurization.

This second stage may be carried out between the end required, the furnace of the second stage is scavenged with a slightly oxidizing current of gas, e.g., one containing a limited quantity of air. The quantity of air -therefore oxygen-to be admitted obviously depends upon the reoxidization of. the product which may be tolerated for substantially the whole oxygen so introduced oxides M00 into M00 This lowers the amount of sulphur but increases the content of free M00 due to a slight reoxidization of the surface of the M00 product.

In another embodiment of my process, the desulphu'rization may also be performed at a temperature comprised between 600 C. and 700 C. However, one must wait until the emission of S0 has become sufficiently low so that the required rate of flow of the diluent gas necessary for maintaining under 10% the partial S0 pressure in the atmosphere of the furnace is not economically prohibitive. In this embodiment, the reaction between thev M00 and the MOS; may continue to a certain extent with the last traces of reducing sulphur in the reaction product. If in this case the gaseous scavenging necessary to lower the concentration of sulphur dioxide is performed with air, the resulting oxidization on the surface of the molybdenum dioxide may even hasten the end of the reaction. Accordingly, very low sulphur contents are attainedin the M00 product, and the termination of the reaction limits the increase in the content of free M00 in the M00 product in comparison with that attained by scavenging with a slightly oxidizing gas at less than 600 C.

The cooling of the product rich in M00, is carried out in a non-oxidizing atmosphere, neutral or reducing, to about 250 C., to prevent its reoxidization. The inert atmosphere employed for the desulphurization stage is particularly suitable.

In one application of my process, the second stage, desulphurization, occurs in the same furnace as the first stage, wherein the atmosphere is modified so that it contains 'less than 10% S0 In another embodiment of my process, the reaction products are withdrawn from the furnace at the end of the first stage and introduced into another enclosure in which are conditions favorable for desulphurization.

Also, the process may be conducted continuously from the mixing to the extraction of the final cooled M00 product. It is further possible to practice the process by forming within an identical apparatus two enclosures separated by a partition wherein the lower extremity is immersed in the product which is being processed.

The following are non-limiting examples of my process.

Example I kg. of molybdic oxide of technical quality, containing 92% M00 and 14 kg. of molybdenite concentrate containing 86% MoS were carefully mixed and then treated in a rotary furnace in which they progressively reached a temperature of 650 C. In this furnace, they were maintained at this temperature for approximately 3 hours under a pressure close to atmospheric pressure. The furnace was equipped with devices for temperature control and atmosphere conditioning. When the atmosphere in the furnace contained 80% of S0 the release of sulphur dioxide had practically stopped. Thereafter, the temperature of the furnace was allowed to drop to 500 C. and the atmosphere in the furnace was diluted with a current of nitrogen in such manner that the concentration of sulphur dioxide was kept below At the end of an hour, heating was stopped, and the inert atmosphere maintained above the product until the temperature dropped to 200 C. The product Withdrawn contained:

Percent S 0.10 M00 520 Example II Percent S 0.34 M00 6.10

This product was then introduced into a second furnace substantially identical with the first and equipped with devices for the conditioning of the atmosphere. There it remained for approximately 2 hours at 600/ 650 C. while subjected to an atmosphere whose sulphur dioxide content was maintained at 10% by blowing in approximately 2 m. of air per 100 kg. of product. The resultant product removed from the furnace at a lower temperature than 200 C. contained:

Percen. S 0.080 M003 Example 111 In this example, I employed an oxide direct from roasting of molybdenite concentrates within a specially controlled furnace. The oxide contained:

' Percent M002 M00 48 M082 2 Insert substances 8 This oxide was mixed with 9 kg. of molybdenite concentrate and treated as in Example I. At the end of the operation, a product was obtained containing:

While I have shown and described preferred embodiments of my invention, it may be otherwise embodied Within the scope of the appended claims.

I claim:

1. A process for the production of molybdenum dioxide comprising:

(A) mixing finely divided products containing stoichiometric amounts of M00 and MoS to form a reaction mixture;

(B) heating said reaction mixture to substantially between 600 C. and 700 C. in a closed chamber to freely evolve S0 and maintaining a pressure in the chamber slightly above atmospheric pressure to prevent air from entering the chamber thereby forming a reaction product having a low sulfur content;

(C) desulphurizing said reaction product in an atmosphere that contains less than 10% S0 and at a temperature substantially between 400 C. and 600 C. to obtain molybdenum dioxide; and

(D) cooling said molybdenum dioxide to at least 250 C. under one of a neutral and of a reducing atmosphere.

2. The process of claim 1 characterized by said M00 being grains smaller than 2 mm.

3. The process of claim 1 characterized by carrying out said heating and maintaining until the atmosphere of a furnace in Which said heating and maintaining is performed is more than 80% S0 4. The process of claim 1 characterized by carrying out said heating and maintaining until the sulphur content of the reaction product is less than 0.5% sulphur and less than 15% M00 5. The process of claim 1 characterized by during said desulphu-rizing subjecting said reaction product to flows of an inert gas to maintain the S0 content of said atmosphere less than 10%.

6. The process of claim 1 characterized by during said desulphurizing subjecting said reaction product to a gas less oxidizing than air.

References Cited UNITED STATES PATENTS 1,637,838 8/1927 Simonds -6 2,398,114 4/1946 Rennie 23--21 X 2,817,583 12/1957 Schlecht et a1. 75-7 2,987,392 6/1961 Supiro 75-9O OSCAR R. VERTIZ, Primary Examiner. H. T. CARTER, Assistant Examiner. 

1. A PROCESS FOR THE PRODUCTION OF MOLYBDENUM DIOXIDE COMPRISING: (A) MIXING FINELY DIVIDED PRODUCTS CONTAINING STOICHIOMETRIC AMOUNTS OF MOO3 AND MOS2 TO FORM A REACTION MIXTURE; (B) HEATING SAID REACTION MIXTURE TO SUBSTANTIALLY BETWEEN 600*C. AND 700*C. IN A CLOSED CHAMBER TO FREELY EVOLVE SO2 AND MAINTAINING A PRESSURE IN THE CHAMBER SLIGHTLY ABOVE ATMOSPHERIC PRESSURE TO PREVENT AIR FRO ENTERING THE CHAMBER THEREBY FORMING A REACTION PRODUCT HAVING A LOW SULFUR CONTENT; (C) DESULPHURIZING SAID REACTION PRODUCT IN AN ATMOSPHERE THAT CONTAINS LESS THAN 10% SO2 AND AT A TEMPERATURE SUBSTANTIALLY BETWEEN 400*C. AND 600*C. TO OBTAIN MOLYBDENUM DIOXIDE; AND (D) COOLING SAID MOLYBDENUM DIOXIDE TO AT LEAST 250*C. UNDER ONE OF A NEUTRAL AND OF A REDUCING ATMOSPHERE. 